The present invention relates to a flux for soldering, and particularly to a flux for use in soldering using solder preforms, such as ball solder, washer solder, and pellet solder.
Recently, solder preforms have been used widely in soldering electronic devices. Ball solder, for example, is used to form bumps for ball grid array (BGA) packages, washer solder is used to solder lids of semiconductor packages, and pellet solder is used to fix IC devices to substrates.
When a flux and solder preform are used to carry out soldering, a flux is first applied to a predetermined area of a substrate, a solder preform is then placed on it, and the resulting soldering assembly is conveyed into a heating apparatus such as a reflow furnace wherein soldering is completed.
Conventional fluxes for such use comprise rosin and an activator dissolved in a low melting point solvent (referred to below as a "low melting solvent") such as isopropyl alcohol (IPA) or 2-ethoxy ethanol.
When soldering is carried out using solder performs, the solder preform must never slip off its desired mounting position, and it must never be moved from the mounting position. In the case of soldering of BGA packages, for example, there are several hundred lands on each of which ball solder must be placed. Thus, if one of several hundred pieces of ball solder is removed from its assigned land, the BGA device, after soldering, will not function, and a quite expensive BGA device must be wasted. This is because all the soldering lands must be connected to their own destinations through respective pieces of ball solder.
However, during soldering, solder preforms easily slip off and move from their respective lands, such disadvantage being collectively called "displacement". Especially when soldering is carried out using ball solder in a reflow furnace, displacement of ball solder from the assigned lands takes place frequently. This is because ball solder is round and can easily roll around. In addition, since a substrate on which ball solder has been placed is often inclined and the substrate is sometimes subjected to mechanical shock, during the time when the substrate is transferred to a conveyor passing through the reflow furnace, displacement of ball solder quite easily occurs and is almost inevitable under usual conditions. When the substrate is subjected to vibration, too, such displacement occurs easily.
Furthermore, electronic devices to which soldering with solder preforms is applied must be free from flux residue because flux residue which remains after soldering not only decreases the insulation resistance of these electronic devices but also causes malfunction of the device due to the formation of a corrosion product. Thus, it is necessary to thoroughly wash off flux residue after soldering of these electronic devices.
For this purpose, it was conventional to use cleaners such as Fron (tradename) and trichloroethylene, which can dissolve rosin efficiently, to wash an electronic device or substrate after soldering. However, these cleaners cause serious environmental problems in that they contribute to destroy of the ozone layer as well as to pollution of underground water. Therefore, recently alcohols have been used widely instead of the before-mentioned cleaners, since the use of alcohols does not cause such environmental problems.
Thus, a flux for use in soldering using solder preforms must be washed off with alcohol.
A flux is sometimes stored for a substantial period of time before use. During storage the tackiness of a flux is gradually degraded. If the tackiness can be maintained at constant even after a substantial period of storage time, it would be quite easy to use such cleaners.
In a production line, after application of a flux to a substrate, the substrate is sometimes left overnight without solder preforms being placed on it. For example, when the application of a flux is performed just before the end of working hours and the placement of solder preforms must be put off until the next day, it would be advantageous under these situations for the tackiness of the flux to be maintained until the next day.
Thus, not only must a flux for use in soldering using solder preforms exhibit a strong tackiness, but also such strong tackiness must be maintained for a longer period of time even during storage or after the application of the flux to a substrate, for example.